The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
This invention relates to radio frequency (RF) systems used for navigational purposes, and, more particularly, to aircraft navigation systems that utilize Global Positioning System (GPS) signals and include anti-jam devices.
Satellite navigation systems employ a plurality of satellites which generate respective signals for reception by an RF receiver on an airborne or surface vehicle, or other platform for computation of the platform position. A navigation control can thus steer the platform on the basis of a stream of position calculations. Well known satellite navigation systems include the Global Positioning System (GPS) and the Global Navigation Satellite System (GLONASS). As used herein, the term Global Position System or GPS refers generically to satellite navigation systems including the Global Position System, GLONASS, and any additional satellite navigation system which may be created in the future.
The Global Positioning System is a network of satellites that transmits information in signals, which when received and decoded, allow a receiver to accurately fix its position within a coverage area. The global positioning system (GPS) satellite navigation system provides a worldwide position, velocity and time reference available for utilization in electronic navigation systems. The utility of the Global Positioning System for guidance applications is well recognized.
Unfortunately, GPS guidance, which uses low power signals from satellites in an 11,000 nautical mile orbit, is notoriously easy to jam with low power, low cost jammers positioned at various locations. Since a large number of jamming units could be located in a given area, one by one elimination of the jammers may not be feasible. If only a few remain operational, they can effectively jam the GPS signals.
In addition, in an environment with strong RF fields, interference signals may have jamming effects on an RF receiver even though they are not intentionally generated as jamming signals. Jamming interference signals may originate, for example, from a radio or television tower, or any high frequency device such as a radar, a radio, or a cellular phone.
Accordingly, the term xe2x80x9cjamming signalxe2x80x9d as used herein refers to both jamming and interference signals. In addition, the term xe2x80x9cjamming freexe2x80x9d includes xe2x80x9cinterference-freexe2x80x9d, and the term xe2x80x9canti-jam capabilityxe2x80x9d includes xe2x80x9canti-interferencexe2x80x9d capability.
Several measures have been suggested to provide an anti-jamming countermeasure for GPS guided vehicles. One measure involves the use of a nulling antenna system. In accordance with that approach, the vehicle is equipped with a number of antennas which point a null in the antenna radiation pattern at the source of the jamming signal. Though somewhat effective, this approach can be expensive as the number of jammers that can be nullified is one less than the number of antenna elements. Hence, that approach may not be practical if a large number of inexpensive jammers are used.
Other techniques have been proposed to address the problem of interfering signals that lie within the GPS bandwidth. One approach is to use an increased number of bits in the ADC (A/D converter) and rely on the correlation process to mitigate the impact of the jammer. This approach is based on the premise that the correlator will distinguish between the Gold code and a CW (tone), provided that the power level of the jammer is not so large as to send the ADC into a saturation mode. By increasing the dynamic range of the ADC (with the concurrent increase in number of bits) the effects of jamming signals can be reduced.
Another approach is one of xe2x80x9ccancellationxe2x80x9d rather than xe2x80x9cremoval.xe2x80x9d That is, a xe2x80x9creplicaxe2x80x9d (or as close to a replica as is feasible) of the jamming signal is generated and is subtracted from the received signal. This cancellation method is of value when the jamming signal is indeed a very narrow-band CW signal, essentially a pure tone. For wider applicability, multiple CW xe2x80x9creplicasxe2x80x9d can be generated and subtracted in the event that there are more than one jamming signals.
While an example of an anti-jam GPS navigation system is disclosed in U.S. Pat. No. 5,955,987, there is a need for a system and method for navigating air vehicles that provides improved resistance to jamming signals.
A navigation system for an unmanned aerial vehicle constructed in accordance with this invention comprises a first antenna for receiving a global positioning system signal, a first interference suppression unit coupled to the first antenna, the first interference suppression unit suppressing interference in the global positioning system signal using a first interference suppression technique, a first navigation unit for receiving signals from the first interference suppression unit, a second antenna for receiving the global positioning system signal, a second interference suppression unit coupled to the second antenna, the second interference suppression unit suppressing interference in the global positioning system signal using a second interference suppression technique, and a second navigation unit for receiving signals from the second interference suppression unit.
The system can further comprise a third navigation unit for receiving signals from the first interference suppression unit, and a fourth navigation unit for receiving signals from the second interference suppression unit.
The first interference suppression technique can comprise an antenna null steering technique, and the second interference suppression technique can comprise a wavefront polarization difference technique.
The first antenna can comprise a controlled radiation pattern antenna.
The invention also encompasses a method for controlling the flight of an air vehicle, the method comprising the steps of using a first antenna to receive a global positioning system signal, suppressing interference in the global positioning system signal using a first interference suppression technique to produce a first interference suppressed global positioning system signal, operating a first navigation unit in accordance with the first interference suppressed global positioning system signal, using a second antenna for receiving the global positioning system signal, suppressing interference in the global positioning system signal using a second interference suppression technique to produce a second interference suppressed global positioning system signal, operating a second navigation unit in accordance with the second interference suppressed global positioning system signal, and using at least one of the first and second navigation units to control the flight of the air vehicle.
The method can further comprise the steps of operating a third navigation unit in accordance with the first interference suppressed global positioning system signal, operating a fourth navigation unit in accordance with the second interference suppressed global positioning system signal, and using at least one of the first, second, third or fourth navigation units to control the flight of the air vehicle.
The first interference suppression technique can comprise an antenna null steering technique, and the second interference suppression technique can comprise a wavefront polarization difference technique.
The first antenna can comprise a controlled radiation pattern antenna.